The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output voltage on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side but this adds to cost and control complexity. Thus, primary-only feedback techniques have been developed that use the reflected voltage on the primary side of the transformer in each switching cycle.
In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off. The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques use this reflected voltage to efficiently modulate the power switching and thus modulate the output voltage.
The reflected voltage not only provides feedback information but is also used to power the controller that controls the cycling of the power switch. For example, the reflected voltage may be rectified and filtered across a controller power supply voltage capacitor to produce a power supply voltage for the controller. This usage of the reflected voltage to power the controller presents a problem, however, during low-load or no-load periods of operation. This problem may be better understood with reference to FIGS. 1A through 1E. FIG. 1A illustrates how the load current may suddenly shut off in response to, for example, a user disconnecting a portable device from a switching power supply. The load voltage will then slowly decline within a regulation envelope as shown in FIG. 1B. The corresponding power switching cycles are shown in FIG. 1C, which illustrates the cessation of the cycles upon the removal of the load. Although the power switch has stopped cycling, the controller current is essentially constant as shown in FIG. 1D. Since the controller continues to burn power despite the lack of switching, its power supply voltage may fall out of regulation as shown in FIG. 1E. In that regard, the controller receives its power supply voltage through rectification of the reflected voltage pulses. But such pulses are not generated if the power switch is not cycling. Since the controller current may remain constant, the controller power supply voltage may thus fall relatively rapidly out of regulation, which then triggers a shutdown and reset of the controller. To alleviate this problem, one solution is to over-design the controller power supply voltage capacitor. But such a solution raises costs.
Accordingly, there is a need in the art for improved regulation of the controller power supply voltage for flyback converters.